It's a market that has long been given the cold shoulder by retailers and designers alike.
But recently, these two industries have turned an eye toward the often ignored plus-size apparel category, generating a 7 percent sales increase and lifting the category to $17.6 billion in revenue, according to The NPD Group.
The growth has been spurred by traditional retailers, such as H&M and Abercrombie & Fitch, adding higher sizes into their assortments, and by new competitors in the plus-size specialty market, which typically serves women sizes 14 and higher, according to IBISWorld.
It also comes as a study released by the Institute for Health Metrics and Evaluation earlier this week found that nearly 30 percent of the global population is obese.
"It's a huge market and it's totally underserved," said Susan Gregg Koger, co-founder and chief creative officer of ModCloth.
The online retailer, known for its indie and vintage products, was designed in 2002 as a store that catered to women of all sizes. But Gregg Koger, who started the company with her now-husband Eric, found the biggest challenge in achieving this goal came from the fact that the majority of its vendors didn't provide styles in these sizes.
Back in 2012, when ModCloth really began to examine its possibilities for the plus-size market, only 35 of its approximately 1,000 vendors had the operations in place to create plus sizes, Gregg Koger said.
"It was a very, very small portion of the designers who were out in the market who were capable of doing plus sizes," she said.
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Although many times the industry has shied away from creating larger sizes, fearing this would somehow alienate their traditional customer, Gregg Koger said this wasn't always the reason designers weren't making these clothes. Designing plus-size clothing requires more than simply scaling a medium into a larger size, as intricacies such as patterns can play out differently in larger sizes. The result: Many vendors didn't have the know-how or capabilities in-house to serve this customer.
To remedy this, ModCloth hired an in-house fit team to partner with designers who were interested in creating plus-size apparel.
The initiative helped the company more than quadruple its plus-size vendor base to 150 by the end of 2013. Coupled with the launch of an in-house private label collection, the retailer now carries more than 1,000 unique plus-size items. With sales growth of more than 240 percent last year, it's the site's fastest-growing category.
"I didn't realize how radical [this] was going to be until I really started digging into this market," Gregg Koger said. "It's a really gratifying thing to be able to do."
Another online business that's zeroing in on the plus-size market is Gwynnie Bee, an online-only subscription website that allows women to rent clothing.
The service offers plans that vary from checking out one item at a time each month, for $35 a month, to 10 items out at a time, for $159 a month. If women fall in love with an item, they also have the option to purchase it. The site launches new arrivals a few times each week, and houses more than 2,000 total styles, CEO Christine Hunsicker said.
"In a rotational model, the only question that you have to answer is, 'Do I like it enough to try it?'" Hunsicker said.
Although Gwynnie Bee does not disclose its membership figures, Hunsicker said its subscription base has grown 10 to 15 times in the last 10 months. What's more, over 50 percent of users who test out the site through its free trial service join the site.
But the store is not just about experimentation—it's also about serving a customer who often prefers to shop online, and whose size swings more dramatically than that of thinner women.
"Most women have multiple sizes in their wardrobe because there is a decent amount of weight fluctuation throughout the year," Hunsicker said. "You can use the service as you're moving up and down the spectrum."
Despite their target shopper's penchant for online, bricks-and-mortar stores aren't excluded from the plus-size boom. Retailers such as Target and Wal-Mart offer larger sizes, while Ascena Retail Group's Lane Bryant and Catherines stores have the largest hold on the market in the specialty store industry.
Lane Bryant has been working to freshen its image among plus-size shoppers. Most recently, it collaborated with fashion designer Isabel Toledo on a line of clothing, and announced an upcoming lingerie tie-up with designer Sophie Theallet.
It's a change of pace that led the brand to an 8 percent same-store sales gain in the quarter ended Jan. 25, and sales of $278.7 million.
"[Our customer] wants the confidence that all women want. She wants the fashion that all women deserve," Lane Bryant CEO Linda Heasley told CNBC earlier this year.
"Our responsibility is to put the fashion industry on notice," she said.
—By CNBC's Krystina Gustafson.